Suriname P

The Brownsberg mountain is a 35 km long and 10 km wide ridge, oriented along a N-S axis. It is part of the Guiana Shield, an old relict of Precambrian continental crust which extends in Suriname, Guyana, French Guiana, Venezuela and norther Brazil and forms the northern part of the Amazon craton. This region has remained in equatorial position since the early Cretaceous after the opening of mid-Atlantic Ocean so has been subject to a warm and humid climate for at least 140 Myr. This climate conducted to the formation of deep and cation-depleted weathering profiles called laterites and characterized by the development of an iron crust atop the profile. Over 140 Ma, multiple episodes of laterization and physical erosion has led to the formation of dissected high plateaus.
(A) Elevation map of Suriname with bauxitic plateaus highlighted (Monsels and van Bergen, 2017) and (B) a geological map of the Suriname, simplified from Daoust et al. (2011) and Kroonenberg et al. (2016).
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Two field campaigns were performed in 2017 and 2018 to collect the “Leo Falls” lateritic profile along a footpath from the unweathered bedrock (345 m elevation) to the bauxitic cap (465 m elevation). The 120-m thick weathering profile is developed on the 2.2 Ga Greenstone Belt formation, which consists in chloritoschist (mostly composed of chlorite and quartz). The typical lateritic facies were all sampled form the bedrock, the weathered bedrock rich in clays, the mottled zone, the iron duricrust to the terminal bauxitic cap. Mineralogical and geochemical characterization of the lateritic profile were combined with isotopic measurements (d18O, d7Li and d30Si) and dating techniques of clays and iron oxides (EPR and (U-Th)/He methods, respectively). First results have shown that the saprolite has formed during a discrete weathering episode (9-6 Ma) while iron oxides are the heritage of successive dissolution and precipitation processes with ages recorded between 10 Ma and present. Large and unusual enrichment in vanadium, a transition metal, is also documented in the most weathered part of the profile and associated with iron oxide coprecipitation.
Further iron crusts were collected in the area in order to obtain supplementary ages and constrain the timing of laterite formation. These crusts recorded an older episode of lateritization at ~30 Ma, preserved from subsequent dissolution/reprecipitation event, with ages ranging between 34 Ma to 2 Ma.
Reconstructed weathering profile of Leo Falls showing the position of collected samples within the profile (white diamonds). Photographs of representative samples for each unit of the profile (bedrock, saprock, nodular zone and duricrust) are reported above and below the sketch.
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In order to answer these questions, a regolith sampling mission was organized in partnership with the Malawian Geological Survey, the University of Rennes 1 and Total. All the methodologies and approaches of the RECA project will be used on the sampled East African laterite and duricrust samples.

This project will throw another light on the geological history of the Southern African landscape, either by revealing the age of exposure of the high planation’s surfaces from this region and by analyzing the main periods of formation and evolution its tropical regoliths. It will also provide clues to understand the factors that can promote the preservation of old high reliefs through the geological times, a phenomenon also observed on other continents such as South America or Asia. Moreover, this project will also open doors to futures studies that will explore the age of exhumation of high plateaus from other countries of Southern Africa.

Cited references

Daoust et al. (2011). Geological setting of the Paleoproterozoic Rosebel gold district, Guiana Shield, Suriname. Journal of South American Earth Sciences. 32, 3, 222-245
Monsels and van Bergen (2017). Bauxite formation on Proterozoic bedrock of Suriname. Journal of Geochemical Exploration. 180, 71-90
Kroonenberg et al. (2016). Paleoproterozoic evolution of the Guiana Shield in Suriname: A revised model. Netherlands Journal of Geosciences. 95, 4, 491-522.